Images are typically stored in a memory representing tone values for each image pixel of the original image. For a black and white image, the stored image pixels represent the gray scale value corresponding to each pixel. For a color image, each color plane is stored as an array of image pixels each representing the tone value for each pixel of the image in each respective color plane. For example, if each of the pixels of a black and white image is represented by a 8 bit digital word, then the tone value for a given image pixel may be one of 256 values between the black level and the white level.
Continuous tone images do not print well on most printing devices where typically the absence or presence of the ink (or other marking indicia) on the paper is used to represent the printed image. In order to represent halftones (shades between the presence or absence of the printed ink), the original image is screened to produce a pattern, such as variable size dots which appear to the human eye as a halftone image.
In order to prepare a photograph for printing, it is first necessary to perform the step of halftone screening, which converts the continuous grayshades of the original into halftone dots of varying size and shape. Typically, these halftone dots are arranged on a regular grid of approximately 100 dots per inch. This spatial frequency is known as the screen ruling. Thus, one square inch of the final printed photograph will be composed of approximately 10,000 dots.